The processing of semiconductor wafers and other microelectronic components has become of great economic significance due to the large volume of such circuits and components being produced and the significant value associated with them. Competitive pressures have driven dramatic changes in production. Among the most dramatic changes is the reduction in size of the various features of the circuits and components which make up the transistors and other devices being formed. This reduction in feature size has been driven by the need to achieve greater levels of integration, more sophisticated and complex circuits, and reduction in production costs by, for example, obtaining more circuits on each semiconductor wafer or other substrate being processed.
Even though feature sizes used in integrated circuits and other microelectronic components have decreased dramatically, additional reductions are continuously being pursued. As feature size decreases, the importance of accurate temperature control during processing increases to even a greater degree. The temperature at which semiconductor wafers and other substrates are processed has a first order effect on the diffusion of dopants, deposition of materials or other thermal processes being performed. Thus it is important to have processing equipment which can achieve accurate temperature control to meet desired thermal processing specifications.
Temperature control feedback problems encountered in thermal processing of semiconductor devices can be thought of in several different ways. One control problem involves matching the workpiece temperature to the processing "recipe" set by the user. The "recipe" comprises the set-point temperatures, temperature process durations, temperature ramp rates, etc., that define the overall thermal process to which the wafer or other substrate is to be subjected. This recipe is generally programmed by the user and is dependent on the particular thermal processing requirements needed to produce the end product. Each portion of a recipe can conveniently be thought of in terms of three different phases. One phase is a ramp-up phase wherein the operating temperature increases or ramps from a lower level set-point temperature to a higher level set-point temperature. A further phase is a ramp-down phase wherein the operating temperature decreases from a higher level set-point temperature to a lower level set-point temperature. The temperature ramp-up or ramp-down phase is thereafter typically followed by a period during which a desired constant set-point processing temperature is maintained. Such a constant temperature phase includes a stabilization period during which the changing temperature ramp ends and a constant or near constant temperature is achieved. Constant temperature, ramp-up, and ramp-down phases may occur one or more times in a processing cycle. Ultimately, the temperature control problem involves both achieving the desired recipe temperatures and achieving relatively consistent temperatures from one production run to another.
Whether simple or more complex temperature recipes are used, each phase of the process may further be complicated by the introduction of one or more supplementary processing gases or vapor phase processing constituents which affect temperature and thermal response. Such supplementary processing gases are typically gases containing dopants, deposition materials or steam.
Various temperature control problems must be addressed by a thermal process control system if it is to meet the increasingly stringent requirements of the microelectronic circuit manufacturing industry. For example, each wafer in a batch should be subject to the same temperature conditions over the entire thermal processing cycle. Left uncontrolled, temperature variations occur between the wafers positioned near the ends of an array of wafers held within the processing furnace when compared to the wafers disposed at mid-portions of the furnace. There may also be other less predictable variations from wafer to wafer, such as along the array of wafers contained within the processing array.
A still further temperature control problem is associated with temperature variations that occur across the width of an individual wafer or other workpiece being processed. Heat from heating elements disposed about peripheral edges of the workpieces is radiated through the processing vessel. Variations can occur with regard to the heat gain experienced by the peripheral edges of the wafer as compared to the interior areas of the wafer. Variations in the degree of radiant heat transfer and radiant shadowing which occur from wafer to wafer further exacerbates this intra-wafer problem.
Minimization of the overall thermal processing time is also a concern that a thermal processing controller should address. Minimizing the processing time will typically increase the ramp-up phase temperature change rate. Conversely, time concerns will also increase the ramp-down phase temperature change rate. Increased rates of temperature change cause greater difficulties in maintaining recipe temperatures during the processes of transitioning between ramp-up and stabilization phases, and between stable temperatures and relatively rapid temperature ramp-down phases.
Traditionally, semiconductor thermal reactors have used Proportional-Integral-Derivative (PID) controllers to control temperature. Recently, a more accurate temperature control model based on H-.infin. control the has been described and implemented in a furnace for use in microelectronic circuit manufacturing. Such control is disclosed and described in International Publication WO98/35531, titled "MODEL BASED TEMPERATURE CONTROLLER FOR SEMICONDUCTOR THERMAL PROCESSORS", which is hereby incorporated by reference.
The latter control system includes a preferred mode of operation in which a dynamic model formulated through empirical testing is used to control the thermal processing cycle based on a recipe that is, for example, entered by the user. The dynamic model is usually based on empirical testing that takes place over a predetermined temperature range. This predetermined temperature range is typically chosen to be centered about the temperatures at which the furnace is to operate most frequently. Other manners of selecting the predetermined temperature about which the dynamic model is formulated may also be employed.
The present inventors have found that the accuracy of a single controller design decreases as the actual furnace temperature or set-point temperatures of the recipe deviate beyond the predetermined temperature range. The use of a single controller design about a single predetermined temperature range may thus limit the use of the furnace to processing recipes in which the set-points are within this temperature range. As the thermal processing steps performed on semiconductor wafers and other substrates become more complex, a need arises for accurate thermal processing control across a wide dynamic range of processing temperatures.
Another temperature control problem involves the handling of hardware failures during execution of a processing recipe set by the user. Traditionally, temperature control systems used for temperature control of thermal reactors drive the reactor to the desired set-point temperature in accordance with a linear ramp function. A graph of the reactor temperature-vs.-time when the reactor temperature is driven in this manner is illustrated in FIG. 1. As shown, the temperature of the reactor overshoots the set-point temperature before the temperature controller it is ultimately able to regulate the reactor temperature at the set-point. Such overshoot may significantly alter the thermal processing of the semiconductor wafers, or other workpieces, in an undesirable manner. Overshoot becomes particularly problematic given the ever increasing demands placed on thermal reactor systems by the advanced processing techniques used in manufacturing sub-micron semiconductor devices.
Another temperature control problem involves the handling of hardware failures during execution of the processing recipe set by the user. During execution of the recipe, the temperature controller generally relies on one or more sensed temperature inputs to generate the requisite control output signals used to control the power supply to the heating elements and, ultimately, to control the temperature of the reactor. When one or more of the sensed temperature inputs is inaccurate due to, for example, a hardware failure of the temperature sensing element, the resulting control output signals are likewise inaccurate.
Temperature control systems used for temperature control of thermal reactors may shut down the reactor operations and, thus, discontinue execution of the recipe upon detection of a hardware failure of a temperature sensing element. This results in a complete shutdown of the reactor and interruption of the thermal processing of the workpieces. Given the strict thermal processing requirements for semiconductor integrated circuits, this interruption may result in the complete loss of the semiconductor wafers under process. Such losses may be very costly, particularly when the semiconductor wafers are in a late-stage of their processing. Similarly, such losses may be very costly when the semiconductor wafers are large (e.g., 300 mm).
A still further probblem is the problem of heating element failure detection and handling. The present inventors have recognized that heating element failures may occur due to various temporary conditions and that the heating element may again operate normally after the temporary condition ceases to exist or is otherwise rectified. In such instances, the heating element may be driven in an improper manner to the desired temperature and thereby damage the workpieces that are being processed or result in a further failure of the heating element (e.g., over-temperature condition).
The present inventors have recognized each of the foregoing problems and have set forth herein a temperature control system for a thermal reactor that addresses each such problem in a unique and efficient manner.